This application is related to Korean Application No. 9948174, filed Nov. 2, 1999 the disclosure of which is hereby incorporated herein by reference.
The present invention relates to a wafer surface evaluation, and more particularly, to methods and systems for measuring the microroughness of a semiconductor wafer surface.
Semiconductor devices are generally fabricated on the surface of an underlying silicon wafer substrate. Accordingly, it is generally desirable for regions on a wafer surface which will be fabricated to become an active region including such devices to have a high degree of surface flatness. The move towards semiconductor devices having a high integration density and a small feature size further increases the desirability of providing a semiconductor wafer having a high degree of surface flatness also referred to as a low microroughness. It is further desirable for semiconductor wafers used in the manufacture of semiconductor devices to have both a generally uniform degree of surface flatness over the wafer and a reduced average size of roughness.
By way of example, with the reduction in size of devices, such as transistors, which are formed on a semiconductor wafer, a thinner gate dielectric film is generally needed between a gate electrode and the wafer to achieve a desired performance of the transistor. More particularly, even though the thickness of the gate dielectric film typically must be reduced, it is desirable to maintain the electrical properties thereof, such as breakdown voltage strength. In order to meet these design requirements, there is a generally a need to minimize the microroughness of the surface of the semiconductor wafer, especially at the interface between the gate dielectric film and the semiconductor wafer. For example, for a gate dielectric film having a thickness of 50 angstroms (xc3x85) to achieve a desired performance, any roughness that is on the order of a few xc3x85 is generally not acceptable and must be removed. To provide such control, methods and systems which provide for effective measurement and controlling of the microroughness of the surface of a semiconductor wafer are desirable.
Atomic force microscopes (AFMs) are often used to measure the microroughness of wafer surfaces. An AFM generally measures the microroughness of a wafer surface by moving an atom-sized measuring probe along the surface of the wafer for detecting microscopic forces, typically Van der Waals forces, exerted between atoms of the wafer surface and the probe, and detecting changes in such forces due to minute differences in the distances between the atoms and the probe. For microroughness measurement using a typical AFM, a predetermined fixed dimension of a sample, for example, 0.1 micrometer (xcexcm) by 0.1 xcexcm, 1 xcexcm by 1 xcexcm, or 10 xcexcm by 10 xcexcm, is determined as a scanning area and several points are detected within the scanning area. The microroughness obtained by the AFM is usually represented by the root mean square (RMS) average of the detected points.
The use of the AFM in measuring the microroughness of a wafer surface may provide a highly accurate result without destruction of the wafer surface. However, the RMS value may vary depending on the dimension of scanning area and the kind of atoms present on the wafer. That is, the results of an AFM measurement may vary according to the dimensions of the scanning area. In addition, because the dimensions of the scanning area are typically very small relative to the wafer size, the measurement result may not represent the microroughness with respect to the entire wafer which may limit the correspondence of the measured results to the overall wafer surface configuration. Further, because this method is typically slow, microroughness measurement by the AFM may not be suitable for real-time (during manufacture) controlling of the microroughness of a semiconductor wafer.
Another existing technique available for measuring the microroughness of a wafer surface is to measure haze levels using a particle counter. Particle counters typically use a light source to reflect light off the wafer surface and measure the scattering of the light resulting from the microroughness of the wafer surface. Such a measure of the scattering of the light may be used as a measure of the microroughness of the wafer surface. Thus, haze levels from a particle counter may be used as an indicator of microroughness using optical methods. A higher haze level generally indicates that the wafer surface is more rough.
In the measurement of haze levels with a particle counter, the wafer surface is typically not in direct contact with a measuring device, and the measuring speed may be faster compared to measuring using an AFM. Typically, for haze level measurement of a wafer surface, a plurality of haze levels are measured for a particular area over the wafer and the average haze level is calculated to represent the haze level for the particular area. In other words, the haze level for a particular area may be expressed as one average value which limits the haze level output with respect to the particular area to a single value. Also, the haze level for a localized area of selected dimensions within the particular area may not be separately characterized by a measurement. Thus, the haze level obtained may not provide a complete characterization of the microroughness of the wafer surface during the manufacture of semiconductor devices.
Embodiments of the present invention provide methods for measuring a wafer surface. A portion of the wafer surface is measured using a particle counter to provide first measurements corresponding to a plurality of points on the wafer surface. A selected area of the wafer surface including one of the plurality of points is measured using an atomic force microscope (AFM) to provide a microroughness measurement of the selected area. The selected area may be a localized area of the portion of the wafer surface measured using the particle counter. The first measurements and the microroughness measurement are provided as a measurement of the wafer surface.
In other embodiments of the present invention, operations further include formatting the microroughness measurement of the selected area as a 3-dimensional image. The 3-dimensional image is provided as a measurement of the wafer surface. The portion of the wafer surface measured by the particle counter may be substantially all of the portion of the wafer surface to be used as active regions. The first measurements may be provided as a measurement of the microroughness of the entire wafer surface and the 3-dimensional image may be provided as a microroughness uniformity measurement of the wafer surface.
In further embodiments of the present invention, the AFM measurement operations are preceded by mapping the selected point from a first coordinate system associated with the particle counter to the selected area in a second coordinate system associated with the AFM. Mapping operations may include determining a coordinate value in the first coordinate system of the one of the plurality of points and identifying, by a plurality of defining second coordinate values in the first coordinate system, a 2-dimensional localized area enclosing the first point. The plurality of second coordinate values in the first coordinate system are converted to corresponding coordinate values in the second coordinate system to define the selected area. The first coordinate system may be a X-Y stage coordinate system of the particle counter and the second coordinate system may be a X-Y stage coordinate system of the AFM.
In other embodiments of the present invention, the particle counter measurement operations include irradiating light onto the wafer surface. Light scattered from the wafer surface is measured. A haze level is calculated over the wafer surface as the first measurements based on a variation in an amount of light scattered from the wafer surface. The measured light may be light scattered from a dark field section of the irradiated light.
In further embodiments, the selected area is about 5 micrometers by 5 micrometers. The AFM measurement operations may include determining either a root mean square (RMS) or a peak-to-valley (P-V) output value as the microroughness measurement.
In yet further embodiments of the present invention, systems are provided for measuring a wafer surface. A particle counter measures a portion, such as substantially the entirety, of the wafer surface to provide first measurements corresponding to a plurality of points on the wafer surface. An atomic force microscope (AFM) measures a selected area of the wafer surface including one of the points to provide a microroughness measurement of the selected area. The selected area is a localized area of the at least a portion of the wafer surface measured by the particle counter. A controller maps the selected point from a first coordinate system associated with the particle counter to the selected area in a second coordinate system associated with the AFM. The controller further provides the first measurements and the microroughness measurement as a measurement of the wafer surface.
In other embodiments of the present invention, methods are provided for measuring the microroughness of a wafer surface. A haze map of the wafer surface is prepared using a particle counter having a X-Y stage coordinate system. A first coordinate value associated with a position of a first point selected from the haze map is determined. A 2-dimensional localized area enclosing the first point is determined. A plurality of second coordinate values which define the localized area are determined. The first coordinate value and the plurality of second coordinate values are converted into corresponding coordinate values on a X-Y stage coordinate system of an atomic force microscope (AFM) by coordinate linking. The corresponding coordinate values enable the X-Y stage coordinate system of the AFM to locate the 2-dimensional localized area enclosing the first point. The microroughness of the wafer surface of the 2-dimensional localized area enclosing the first point is measured using the AFM based on the corresponding coordinate values input to the X-Y stage coordinate system of the AFM.
Using the microroughness measurement methods and systems of the present invention, the microroughness within a localized area of a wafer may be accurately measured and, in various embodiments, may be shown as a 3-dimensional image. Thus, the inventive method may be utilized in the manufacture of high density integrated circuits, where tight and accurate microroughness control is desired.